But the explosion was enough to strike fear in commuters, including a witness who walked away with burns.

"I heard a large bang from the doors on the other side of the train. And this fireball above my head singed all my hair, there were burn marks on the top of my head," the witness said. "Everyone just ran off the train. It was scary."

Police say the man was among about 20 people who suffered some sort of injury from the blast. None of the injuries were reportedly life-threatening.

The incident, however, is being treated as a terrorist attack, one that could have taken countless lives as it occurred at the height of London's morning rush hour.

"The investigation is being led by the Metropolitan Police counter-terrorism command and part of national counter-terrorism policing network. And there are many urgent inquiries ongoing now with hundreds of detectives involved looking at CCTV, forensic work and speaking to witnesses.

"This investigation is supported by our colleagues by MI5 and bringing their intelligence expertise to bear on the case," Mark Rowley, Assistant Commissioner for Specialist Operations of the Metropolitan Police Service, told reporters.

Police did not provide details on any suspects, but U.S. President Donald Trump painted them with his signature broad strokes on Twitter.

"Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive," he tweeted.

In another post, he said: "Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off & use better!"